There is provided a NOx occlusion reduction-type catalyst 32 that is provided in an exhaust passage 13 of an internal combustion engine 10, occludes NOx in exhaust when the exhaust is in a lean state, and reduces and purifies the occluded NOx when the exhaust is in a rich state; an exhaust injector 34 that is provided in the exhaust passage 13 and is positioned further upstream than the NOx occlusion reduction-type catalyst; a NOx-purging control unit 60 that performs NOx purging of reducing and purifying the NOx occluded in the NOx occlusion reduction-type catalyst 32 by lowering the exhaust to a prescribed target lambda by fuel injection by the exhaust injector 34; and a NOx-purging-prohibition processing unit 70 that inhibits performance of the NOx purging in a case where the exhaust cannot be lowered to the target lambda even if the fuel injection is performed at a maximum limit injection amount of the exhaust injector 34.